This work presents the characterization of fibrous reinforcement mats in resin injection molding. The fiber mat characterization involved determining the mat permeability and compressibility. Mold filling experiments were conducted using two or more different fiber types in the mat stack, which created transverse porosity, permeability, and compressibility variations. The effect of these variations was studied by taking flow pressure measurements and observing the progress of the flow front of a non-reactive fluid filling a clear acrylic mold that contained the reinforcement mat stack.

The mechanism associated with mold filling in the manufacture of structural RIM (SRIM) and resin transfer molding (RTM) composites is studied by means of flow visualization and pressure drop measurements. To facilitate this study, an acrylic mold with a variable cavity was constructed and the flow patterns of nonreactive fluid flowing through various layers, types, and combinations of preplaced glass fiber reinforcement mats were photographed for both evacuated and nonevacuated molds. The pressure drops in the flow through a single type of reinforcement (e.g., a continuous strand random fiber mat) and also a combination of reinforcement types (e.g., a stitched bidirectional mat in combination with a random fiber mat) were recorded at various flow rates to simulate high‐speed feeding processes (e.g., SRIM) and low‐speed feeding processes (e.g., RTM). By changing the amount of reinforcement placed into the mold, the permeabilities of the different types and combinations of glass fiber mats were obtained as a function of porosity. It is shown that partially evacuating the mold cavity decreases the size of bubbles or voids in the liquid, but ultimately increases the maximum pressure during filling. The results also show that glass fiber mats exhibit anisotropic permeabilities with the thickness permeability, Kz, being extremely important and often the determining factor in the pressure generated in the mold during filling.

When a resin in injected into the mold in liquid composite molding, the preplaced fiber mat may deform near the inlet gate because of the high momentum carried by the injected fluid. A fiber free region near the gate followed by the fiber mat deformation may emerge. This phenomenon is most likely to occur when the stacked fiber mats have low permeability and the resin has high viscosity. A set of mold filling experiments were carried out using an instrumented metal mold and a small transparent mold to investigate the fiber mat deformation during mold filling. Experimental results showed that the fiber mat deformation was limited to a small region near the gate and that deformation greatly reduced the molding pressure. A forced fiber mat deformation employing a modified gate design was proposed to facilitate mold filling in liquid composite molding.

Three stages involved in the RIM process are impingement mixing of di isocyanate and polyol, filling and curing in the mold, and postcuring in a separate oven to achieve complete conversion of diisocyanate. Traces of water in the polyol react with diiso cyanate, and generate CO2 in a side reaction during mold filling and curing. It may not be possible to postcure the RIM part immediately after curing. During that period, known as floating time, moisture in ambient air reacts with residual diisocyanate generating addi tional CO 2. Part of the CO2 diffuses out during the floating period, and the remaining CO2 evolves during postcuring. If these gases are not completely removed from the part during postcuring, they can "pop up" on the surface in the paint baking oven, creating unaccep table surface quality. This phenomenon is called "outgassing," and is investigated in this work.

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